Methanol is primarily used to produce formaldehyde, methyl tertiary butyl ether (MTBE) and acetic acid, with smaller amounts going into the manufacture of dimethyl terephthalate (DMT), methylmethacrylate (MMA), chloromethanes, methylamines, glycol methyl ethers, and fuels. It also has many general solvent and antifreeze uses, such as being a component for paint strippers, car windshield washer compounds and a de-icer for natural gas pipelines
A major use of methyl acetate is as a low toxicity solvent in glues, paints and a broad range of coating and ink resin applications. Methyl acetate also finds use as a feedstock in the production of acetic anhydride.
Methanol may be produced on a commercial basis by the conversion of synthesis gas containing carbon monoxide, hydrogen and optionally carbon dioxide over a suitable catalyst according to the overall reaction:2H2+CO⇄CH3OH
Widely used catalysts for methanol synthesis from synthesis gas are based on copper.
Methyl acetate may be produced, as described, for example, in WO 2006/121778, by carbonylating dimethyl ether with carbon monoxide in the presence of a zeolite carbonylation catalyst, such as a mordenite zeolite.
The production of methyl acetate by the carbonylation of dimethyl ether may also be carried out using mixtures of carbon monoxide and hydrogen, as described, for example, in WO 2008/132438. According to WO 2008/132438, the molar ratio of carbon monoxide:hydrogen for use in the carbonylation step may be in the range 1:3 to 15:1, such as 1:1 to 10:1, for example 1:1 to 4:1.
WO 01/07393 describes a process for the catalytic conversion of a feedstock comprising carbon monoxide and hydrogen to produce at least one of an alcohol, ether and mixtures thereof and reacting carbon monoxide with the at least one of an alcohol, ether and mixtures thereof in the presence of a catalyst selected from solid super acids, heteropolyacids, clays, zeolites and molecular sieves, in the absence of a halide promoter, under conditions of temperature and pressure sufficient to produce at least one of an ester, acid, acid anhydride and mixtures thereof
U.S. Pat. No. 5,286,900 relates to a process for preparing an acetic acid product selected from acetic acid, methyl acetate, acetic anhydride and mixtures thereof by conversion of a synthesis gas comprising hydrogen and carbon oxides, said process comprising the steps of: (i) introducing synthesis gas into a first reactor at a pressure of 5-200 bar and a temperature of 150-400° C., and catalytically converting the synthesis gas into methanol and dimethyl ether and (ii) carbonylating the methanol and dimethyl ether formed in step (i) by passing the entire effluent from the first reactor to a second reactor and carbonylating therein, at a pressure of 1-800 bar and a temperature of 100-500° C. in the presence of a catalyst, the methanol and dimethyl ether to an acetic acid product.
EP-A-0566370 describes a process for the production of ethylidene diacetate, acetic acid, acetic anhydride and methyl acetate directly from synthesis gas via an intermediate product stream containing dimethyl ether. Dimethyl ether is produced from synthesis gas in a first liquid phase reactor and the reactor effluent comprising dimethyl ether, methanol and unreacted synthesis gas flows to a second liquid phase reactor containing acetic acid in which the oxygenated acetyl compounds are synthesized catalytically. Vinyl acetate and additional acetic acid optionally are produced by pyrolysis of ethylidene diacetate in a separate reactor system. Synthesis gas is preferably obtained by partial oxidation of a hydrocarbon feedstock such as natural gas. Optionally a portion of the acetic acid co-product is recycled to the partial oxidation reactor for conversion into additional synthesis gas.
Synthesis gas comprises carbon monoxide and hydrogen. Optionally carbon dioxide is included. The synthesis gas ratio or stoichiometric number (SN) of a synthesis gas composition is conventionally calculated asSN=(H2—CO2)/(CO+CO2)wherein H2, CO and CO2 represent the composition of the gas on a molar basis.
Desirably, the optimum stoichiometric number of a synthesis gas for use in methanol production is 2.05. Typically, however, processes for the production of methyl acetate by the carbonylation of dimethyl ether with synthesis gas employ synthesis gas with a stoichiometric excess of carbon monoxide. Thus a major drawback in carbonylation and methanol synthesis processes is that the hydrogen:carbon monoxide ratios desirable for methanol synthesis are significantly higher than the desired ratios for carbonylation.
A further drawback of processes for the carbonylation of dimethyl ether is that a purge gas must be removed from the process to prevent recycle components from reaching unacceptable levels in the reactor. Typically, purge gases are disposed of by burning. Purge gas from the carbonylation process contains carbon monoxide and invariably contains some dimethyl ether and methyl acetate. Therefore, the removal of these components by purging represents a loss of values and reduces the overall efficiency of the process.
As described above, processes for the carbonylation of dimethyl ether with synthesis gas typically employ synthesis gas with a stoichiometric excess of carbon monoxide. This results in unconsumed carbon monoxide being withdrawn (together with hydrogen which generally remains unconsumed in the process) from the process as part of the carbonylation product stream. Typically, to avoid loss of carbon monoxide feedstock from the process, it is recycled to the carbonylation reactor together with the unconsumed hydrogen. A disadvantage of this is that hydrogen builds-up in the reactor and an undesirable reduction in the carbonylation reaction rate is observed.
A further drawback is that it has now been found that processes for the carbonylation of dimethyl ether in the presence of zeolite catalysts suffer from the undesirable formation of certain low-boiling by-products, including olefins, for example ethylene and C2 oxygenate compounds, such as acetone and acetaldehyde. Recycling streams containing these low-boiling by-products to the carbonylation process causes a reduction in the carbonylation catalyst lifetime and an increase in the level of by-products formed in the carbonylation process.
Furthermore, due to difficulties associated with the transport and storage of synthesis gas, it is typically generated in situ. Thus, a significant expense for new methyl acetate and methanol production capacity is the capital and operating costs associated with synthesis gas generation.